Light travels in waves that can stretch and compress when moving to and from a stationary observer. Each wave of light can be visualized as a sinusoidal wave. Each sinusoidal wave can be measured by its wavelength and frequency. Wavelength is the distance over which the wave's shape (e.g. sinusoidal) repeats. Wavelength can be measured by the distance between consecutive corresponding points of the same phase of the sinusoidal wave. For example, one can measure the distance between two peaks of a sinusoidal wave. Frequency is the number that a wave's shape (e.g. sinusoidal) repeats per a unit of time. For example, one can measure the number of sinusoidal waves in one minute.
When an object in motion moves away from a stationary observer, light waves are stretched. The stretched light waves exhibit a longer wavelength and lower frequency. This phenomenon is known as “redshift.” Cosmological redshift is observed due to the expansion of the universe. Astronomers are able to use redshift to measure the distance between Earth and objects in space. When an object moves toward a stationary observer, light waves are compressed. The compressed light waves exhibit a shorter wavelength and higher frequency. This phenomenon is known as “blueshift.”